In the area of biomaterials design, two major material characteristics are noted to be of importance. These are the chemical and the mechanical properties of the biomaterial. Chemical properties dictate whether a biomaterial is toxic, carcinogenic, reactive, or degradable within the bio-system. Mechanical properties determine a biomaterial's capabilities as a load-bearing device, tissue augmentation device, or tissue replacement device. It is generally known that the flexure properties of a biomaterial must match closely that of juxtaposed tissue in order for long term implant success to occur. Stiff or rigid implant material joined to softer tissues will cause immediate tissue response including encapsulation of the implant. Of additional importance are the density of the biomaterial, the porosity of the biomaterial, the creep resistance of the biomaterial, and the elasticity characteristics of the biomaterial. Relative porosity allowing permeability of the biomaterial to biomolecules such as albumin, fibrinogen, lipoprotein, and macroglobulin is also important for the long term success of the biomaterial.
Following implantation of most biomaterials, the immediate response of a bio-system, such as the human body, is to expel the biomaterial. Biomaterials can either be extruded from the body or walled-off if the materials cannot be removed. These responses are related to the healing process of the wound where the biomaterial is present as an additional factor. A typical response is that inflammation-activating leukocytes quickly appear near the biomaterial followed by giant cells which try to engulf the material. If the biomaterial is inert enough, foreign body giant cells may not appear near the biomaterial, lowering the overall inflammatory response.
Silicone polymers continue to be used commercially as biomaterials in both short-term and long-term implant devices. The relative chemical inertness of silicone makes it a fairly non-toxic biomaterial for long term implantation with relatively few complications reported. The elastic mechanical property of the silicone polymer allows the material to be used in load bearing applications without appreciable problems of material creep or tearing. While permeable to certain gases such as oxygen and carbon dioxide, silicone polymers are relatively impermeable to biological proteins, fluids or cells.
Open-structure, highly-crystalline porous materials such as micro-porous expanded polytetrafluoroethlyene or ePTFE are used as biomaterials to manufacture medical devices as well. Vascular grafts, soft tissue patches, sutures, ligaments, tissue augmentation membranes, and burn membranes are such devices made of ePTFE. The chemical inertness or non-reactive nature of polytetrafluoroethylene provides for a very non-toxic biomaterial. Expanded PTFE is characterized by its low density of typically less than 1 gm/cc, its high crystallinity, and its fibril and node open structure of average pore size typically greater than 30 micrometers. Implant products made of micro-porous ePTFE are typically designed to allow for cellular infiltration or tissue ingrowth during implantation. While cellular ingrowth is sometimes desirable, complications may result. These complications include but are not limited to infection, increase in implant rigidity leading to tissue compliance problems, and difficulty in removing the implant should the need arise. In addition, the highly crystalline nature of ePTFE presents mechanical compliance problems in many biomaterial applications due to the inherent stiffness of ePTFE medical devices.